Groupon, the company that pioneered online group discounts, saw its stock climb by nearly a third in its public debut Friday, showing strong demand for an Internet company whose business model is considered unsustainable by some analysts. Groupon's stock jumped $6.40, or 32 percent, to $26.40 in late morning Friday after trading began at about 10:45 a.m. Earlier, the stock was trading as high as $31.14. Big fluctuations are common for companies that have just gone public as investors gauge what to do with the stock. The stock is trading on the Nasdaq Stock Market under the ticker symbol "GRPN."